This invention relates a liquid crystal display panel using an optical phase shifting substrate and more particularly to a liquid crystal display panel of improved contrast and reduced angle-of-vision dependency.
The liquid crystal display panel in general comprises a liquid crystal cell and a polarizing sheet disposed on either side of said cell and its basic structure may be represented by the scheme: Polarizing sheet/liquid crystal cell/polarizing sheet
Of these constituent members, the liquid crystal cell comprises a pair of substrates each carrying a transparent electrode layer on one side thereof as disposed face-to-face with a spacer means interposed between the respective transparent electrode layers, with the gap between the two substrates having been filled with a liquid crystal material and the peripheral edges of the assembly sealed off with an epoxy or other adhesive. The schematic of the assembly thus constructed is:
Substrate/transparent electrode/liquid crystal/transparent electrode/substrate
Each of these substrates must be optically transparent and isotropic. Otherwise, the product liquid crystal display panel will be seriously colored and thereby interfere with a visual recognition of the information displayed. Therefore, it is essential that an amorphous material be chosen for the substrates and, for this reason, sheet glass substrates have been used almost exclusively. However, glass substrates are heavy in weight and cannot be thinned beyond a certain limit. Moreover, these substrates are liable to be broken, i.e. of poor impact resistance, and cannot be taken up on a roll or the like for mass production. Recently, therefore, synthetic resin films have come to be used for the substrates.
Liquid crystal display panels are used in quantities today in the display units of office automation devices such as word processors, personal computers and so on, and recently STN (super-twisted nematic) liquid crystals have been developed and used especially for large-sized liquid crystal display panels. In this system, however, the background is colored yellow, green or deep blue and, therefore, provides only a poor contrast. Moreover, there is the problem of angle-of-vision dependency, which means that the contrast deteriorates when the display panel is viewed at an angle. Therefore, this system has not fully satisfied the users' demand for improvement in visual recognizability.
Recently, however, thanks to the various improvements made in the aspect of liquid crystal structure and material, several large-capacity simple matrix liquid crystals with increased levels of blackness with enhanced contrast have been introduced one after another. Also high-contrast, substantially perfect black-and-white display panels and their color versions have been implemented.
Of these newer panels, the most noteworthy is the panel in which the color of yellow, green or deep blue has been cancelled by the use of two STN liquid crystal cells in overlapping relation. In the cell forming the second layer of this panel structure, the orientation of liquid crystal molecules has been twisted in the reverse direction to cancel out the color produced in the first layer (Nikkei Micro Device, August 1987 issue, pp. 36-38 and Nikkei Micro Device, October 1987 issue, pp. 84-88).
However, while the above published simple matrix liquid crystal panel for black-and-white high-contrast displays has been designed to improve the contrast by superimposing a liquid crystal cell on another cell to render the color neutral (gray), the use of two liquid crystal cells of necessity makes the whole liquid crystal panel much heavier and thicker. This results in that the system is contrary to the current trend toward reduced weight and thickness of the panel. Moreover, this display panel gives rise to a color when viewed at an angle, thereby leaving room for improvement in the aspect of angle-of-vision dependency.
It is an object of this invention to provide a liquid crystal display panel which, while satisfying the aforesaid requirements for reduced weight and reduced thickness, has realized a marked improvement in contrast by way of color neutralization and, at the same time, has virtually eliminated the problem of angle-of-vision dependency, thus overcoming the abovementioned disadvantages of the prior art.